This invention relates to a turbine sealing and air removal arrangement which provides for conducting exhaust from both ends of a turbine to a common vacuum header which also exhausts air from a condenser. More particularly, this invention relates to a turbine sealing and air removal arrangement for steam turbines which reduces the oxygen concentration in the condensate being returned to the steam generators, reduces maintenance, increases efficiency and simplifies system arrangement. This invention also relates to a turbine sealing and air removal arrangement incorporating a metallic bellows valve stem seal which is exhausted to a turbine exhaust trunk to minimize the internal pressure of the bellows and prevent catastrophic failure.
Most conventional steam turbine air sealing/condenser air removal systems are based on labyrinth type turbine rotor gland seals and steam jet type air ejectors for exhausting air which leaks into the turbine glands and the condenser. In the interest of minimizing steam consumption by the steam jet air ejectors, two separate exhaust systems are typically used for turbine rotor gland and condenser air exhausting. Two separate systems are required due to the fact that condenser pressure must be maintained as low as possible, e.g., 0.5 to 10 inches Hg Absolute for best steam cycle efficiency, while the outermost turbine rotor glands must be maintained at slightly below atmospheric pressure in order to prevent steam from leaking out of the turbine casing. The turbine glands in such systems also require that sealing steam be provided during start-up and at low power conditions to preclude air from entering the condenser. This sealing steam requires still another piping system to be installed and maintained. This system and the steam supply to the steam jet air ejectors typically require that reducing or pressure regulating valves be used, which unfortunately are subject to steam erosion at the throttling element of the valves. These regulating valves are commonly the source of unplanned maintenance and plant downtime.
The steam sealing system also requires the use of a turbine rotor turning gear that slowly rotates the rotor during start-ups from cold iron and during temporary shut-downs to prevent bowing of the turbine rotor due to differential thermal expansion. The rotor turning gear is another high maintenance item that is also the source of many operator errors for example, admitting steam while the rotor is on turning gear. Operation of the rotor turning gear is reputed to be the cause of over 90% of all turbine bearing wear since the slow rotation of the rotor is insufficient to develop an oil film which, at normal operating speeds, prevents the bearing surfaces from contacting. For the reasons noted above, power generating stations which employ steam turbines have historically required constant attention by at least one skilled operator. This is particularly undesirable in remote steam power applications where small to medium units must be operated in relatively unprotected environments such as petroleum distillation plants. The recent proliferation of small to medium size cogeneration plants has also demonstrated the need for steam equipment which can be operated unattended for months or years with only occasional planned maintenance being required and minimal capitol investment at installation.